FIG. 1 shows the most advanced prior-art selective-black absorber fabricated by Seraphin of the University of Arizona. It consists of multilayers of Si and Ge separated from tungsten and steel by a diffusion barrier of Cr.sub.2 O.sub.3. The Si and Ge, which transmit infrared radiation provide high absorption below 1 .mu.m; the tungsten provides high infrared reflectivity. The Si reflectivity in the solar spectrum is suppressed by an antireflection coating. The optical-reflectivity spectrum of such a composite is shown in FIG. 1(a). The wavelength-selective properties are good but not outstanding. However, an important advantage of these composites is their stability at a high operating temperature, 500.degree. C., for over 1000 hours.
The most popular prior-art selective-black absorber for low temperature operation is Cr-black. Cr-black is presently deposited on metal sheets by electroplating. FIG. 2 shows the optical reflectivities of two Cr-black coatings. One was electroplated on Cu, the other Mar et al. electroplated on Ni. The coating on Cu has excellent selective properties: a solar absorptivity of 94% and an infrared emissivity of 4%. The Cr-black on Ni also has high solar absorptivity, about 94%, but its infrared emissivity is 12%.
Unfortunately, the optical properties of Cr-black on Cu degrade at operating temperatures above 200.degree. C. in air. However, Cr-black on Ni is stable up to 300.degree. C.
The basic properties of Cr-black absorbers are not yet well understood. Initial X-ray diffraction results suggest that the material is a cermet consisting of polycrystalline Cr.sub.2 O.sub.3 and amorphous or extremely fine-grained Cr metal, in which the front surface is almost entirely Cr.sub.2 O.sub.3 and the metal concentration increases toward the coating-substrate interface. If this is the case, the optical properties may be affected by crystallization of the amorphous chromium at elevated temperatures. In addition, at 350.degree. C. Cr oxidizes in air to Cr.sub.2 O.sub.3. To circumvent these problems a cermet system MgO/Au, in which the metal, Au, is not amorphous and does not oxidize; and a Cr.sub.2 O.sub.3 /Cr cermet with antireflective coating have been developed.